Generally, a signal transmission model of a wireless communications system may be based on the following mathematical formula:y=HWx+n,                 where        
x represents a to-be-transmitted signal, H represents a channel matrix and is used to represent channel characteristics, W is a precoding matrix and is used to represent a matrix for precoding the to-be-transmitted data before the to-be-transmitted data is transmitted by using a channel H, n represents noise, and y represents a signal received at a receive end. The precoding matrix W is usually determined through measurement and feedback. For example, a process of determining a precoding matrix for downlink data may be as follows: A transmit end sends one or more reference signals; and a receive end measures the reference signals, and the receive end determines one or more precoding matrices based on measurement results of the reference signals. The receive end may send a precoding matrix indicator PMI to the transmit end based on the measurement results.
In an existing communications system and a next generation communications system, a multi-stage PMI feedback mechanism may be defined to reduce feedback overheads. To be specific, a precoding matrix W is a product of a first-stage feedback matrix W1 and a second-stage feedback matrix W2. W is a matrix with M rows and R columns. W1 is a first-stage precoding matrix, and the matrix satisfies the following block diagonal structure:
      W    1    =            [                                                  X                              (                k                )                                                          0                                                0                                              X                              (                k                )                                                        ]        ∈          C              M        ×        2        ⁢                                  ⁢        L            
Each diagonal block matrix X(k) is a matrix with M rows and L columns, and two diagonal blocks respectively correspond to ports in two polarization directions at a transmit end. X(k) is determined by the kth vector group or beam group selected in first-stage feedback, and a vector in each vector group is selected from a predefined vector set. W2 is a matrix with 2L rows and R columns, and is used to select a vector from X(k) for each column of W, and determine a phase difference between the two polarized antenna ports. W1 is a wideband parameter, to be specific, has a same value on all subbands within bandwidth used for communication between the transmit end and a receive end. W2 is a subband parameter, to be specific, may have different values on different subbands within the bandwidth used for communication between the transmit end and the receive end. W1 and W2 are indicated by a first PMI 1 and a second PMI 2, respectively. After receiving a reference signal, the receive end reports a PMI 1 and a PMI 2 to the transmit end. The transmit end may respectively determine corresponding W1 and W2 based on the received PMI 1 and PMI 2, to determine a final precoding matrix. The transmit end may alternatively self-determine W1 and W2, to determine a final preceding matrix. Because some vectors in the predefined vector set cause relatively strong interference to a receive end in a neighboring cell, the transmit end may restrict, as required, a preceding matrix set that can be selected by the receive end when the receive end feeds back a preceding matrix. For example, available vectors in the predefined vector set may be restricted, and therefore the PMI 1 fed back by the receive end can only be a PMI 1 corresponding to an unrestricted vector. A restricted vector may be notified by the transmit end may to the receive end by using a bitmap (bitmap) in a field, where bits in the bitmap are in a one-to-one correspondence with vectors in the predefined vector set. The field that includes the bitmap is carried in signaling sent by the transmit end.
However, in the next generation communications system, a parameter type corresponding to the precoding matrix W1 varies with R. For example, when R=1, 2, 5, 6, 7, or 8, a parameter corresponding to W1 includes only a vector, and therefore a quantity of bits in the bitmap only needs to be greater than or equal to a quantity of vectors in the predefined vector set. When R=3 or R=4, in addition to a vector, W1 further corresponds to an additional phase difference, the additional phase difference is selected from a predefined phase set, and the additional phase difference is a parameter different from the phase difference in W2. In this case, a combination of each vector and each phase difference needs to be jointly restricted in the bitmap, and therefore a quantity of bits in the bitmap is greater than or equal to a product of a quantity of vectors in the predefined vector set and a quantity of phase differences in the predefined phase set. Therefore, due to a plurality of bitmaps, a length of the field is greatly increased, and consequently signaling overheads are increased and air interface resources are wasted.